The present invention relates to an improved thermosensitive recording material comprising a support material and a thermosensitive coloring layer formed thereon, which thermosensitive coloring layer comprises a colorless or light-colored leuco dye and a color-developer which induces color information in the leuco dye when heat is applied thereto. More particularly, it relates to a thermosensitive recording material of the above-mentioned type in which as the color developer a particular thiourea derivative is employed.
A conventional thermosensitive recording material comprises a support material, such as paper, synthetic paper or a plastic film, and a thermosensitive coloring layer formed thereon which consists essentially of a composition which is colored upon application of heat thereto. On the thermosensitive recording material, colored images are formed by application of heat by use of a thermal head or a thermal pen, or by application of laser beams thereto. Recording materials of this kind are widely used for making copies from books and documents or as output charts for computers and facsimile apparatus, tickets for vending machines, labels and other charts for recording, since in comparison with other recording materials, they have the advantages that complex steps for recording, such as development and image fixing, are unnecessary, recorded images can be quickly obtained by a comparatively simple apparatus, no noise is generated during operation, and they are free from a problem of air pollution and cheaper than other recording materials. A thermosensitive composition for color formation employed in such a thermosensitive recording material is generally composed of a coloring agent and a color developer which is capable of inducing color formation in the coloring agent upon application of the heat thereto.
As the coloring agent, for example, colorless or light-colored leuco dyes having lactone rings, lactam rings or spyropyran rings are employed.
As the color-developer, a wide variety of acidic materials such as organic acids and phenolic materials are employed. The recording materials in which such a coloring agent and a color developer are used in combination have the advantages that the color tone of the images is clear, the whiteness of the background is high and the images are excellent in weathering resistance. Therefore they are widely used.
Recently other conventional recording systems are being replaced by the thermosensitive recording systems and a demand for the thermosensitive recording systems is increasing. In accordance with this trend, further improvement of the quality of the thermosensitive recording materials for use with the thermosensitive recording systems is desired. In particular, a thermosensitive recording material which is resistant to oils and finger prints is desired. As the materials which have effects on the quality of the thermosensitive recording materials, there are a coloring agent, a color-developer, a binder agent which binds or protects the coloring agent and the color developer, and other additives such as a dispersing agent and a pigment. Of the above-mentioned agents, the color developer has the most significant effect on the quality of the thermosensitive recording materials.
As a conventional color-developer, Bisphenol A is in general use, since its color development performance is excellent. Bisphenol A, however, has the shortcoming that images developed by Bisphenol A fade or disappear, upon coming into contact with oils, such as olive oil or salad oil, and are also vulnerable to finger prints. This shortcoming of Bisphenol A is known to those who are skilled in this art.